Compass use

[AzAtty]

"if you pick out headings and distances on the surface using the lensatic, and then duplicate that pattern on the dive" Yep that's what I had in mind

Then fire away. I think that it would be a great way to map a dive site--drop some buoys with a few chemlights on the downline to mark the boundaries, take the bearings from shore, and, if you want to be super accurate, hit them with a laser rangefinder. Some basic geometry would ensure the shape of your site, and you could easily set up a grid pattern over the site. Dive each grid and note the depth, and you'd have a great topo of the site. Good luck with it.